This invention relates generally to a speed averaging circuit and more particularly to such a circuit for use in controlling a plurality of adjustable speed electric motors on an electrically propelled traction vehicle.
In an electric propulsion system for a large traction vehicle such as a diesel-electric locomotive or rapid transit car, a plurality of adjustable speed electric motors are respectively coupled in driving relationship to two or more axle-wheel sets of the vehicle. Suitable means is provided for controlling all of the motors so that they operate in unison to propel (or to retard) the vehicle. For proper operation, the motor controls are conventionally supplied with certain command and feedback signals, including feedback signals representative of the revolutions per minute (RPM) of the traction motors. In practice the rotational speeds of the respective motors may differ from one another due to small differences in wheel diameters, or due to a temporary wheel slip (or slide) condition that is not shared equally by all of the axle-wheel sets.
Others have heretofore disclosed circuits in which a plurality of different speed signals are so processed that the lowest (and/or highest) speed is determinative. See U.S. Pats. Nos. 3,871,235; 4,006,417; and 4,031,509. In the present invention, a circuit is provided for combining different speed signals so as to obtain a single feedback signal that represents their average.